Low-temperature plasma source



March 14, 1961 w. KNAUER 2,975,320

LOWTEMPERATURE PLASMA SOURCE Filed Dec. 3, 1958 INVENTOR. Wu LFEANE KNAUE-H rralen iy LOW-TEMPERATURE PLASMA SOURCE Wolfgang Knauer, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Dec. 3, 1958, Ser. No. 777,952

4 Claims. (Cl. 313-187) This invention relates to apparatus and a method for providing a space-charge-neutralized electron stream and particularly to apparatus and a method for providing a low-temperature plasma stream.

In various applications using electron streams, it is desirable that the electron stream be emitted at a relatively low temperature. In some cases, especially where the electrons are not to be accelerated but rather are to drift at thermal velocities, it is desirable that positive ions be provided at the electron emitter in order to neutralize the space charge of the electrons. For example, in order to provide a versatile thermionic converter diode (i.e., a device to convert heat energy directly to electrical energy), it is desirable to obtain electron emission at a relatively low temperature. At the same time it is preferred that the electron stream be space charge neutralized by the presence of positive ions. Moreover, in some applications, low-temperature emission is also desired for the purpose of its attendant low-noise characteristics.

It is known that in normal thermionic emission of electrons, the electrons are emitted with a spread of thermal velocities, and that this velocity spread manifests itself as undesirable noise. It is also known that the higher the emission temperature the greater the thermal velocity spread and, correspondingly, the greater the noise due to random thermal velocities. Moreover, it is known to neutralize the space charge repulsion forces of an electron stream by supplying positive ions thereto. The positive ions may be supplied by the phenomenon of contact, or resonance, ionization at the emitter surface. However, due to the inherent properties of suitable materials, the prior art has held to the tacit belief that it was not possible to provide both low temperature electron emission and contact ionization from the surface of a single element. Consequently, the prior art has failed to teach the provision of a practical low-temperature plasma source (i.e., a low-temperature source of an ion-neutralized electron stream) having the attendant advantage of low-noise characteristics. a

It is therefore an object of this invention to provide a novel and improved low-temperature plasma source, and particularly to provide an improved plasma source wherein electrons are thermionically emitted at a relatively low temperature with corresponding low thermal noise properties and are space charged neutralized by ions produced at the emitter surface by contact ionization.

Briefly, according to my invention a source element comprises surface portions having a given work function suitably interspersed with other portions having a substantially lower work function. Means such as a heater coil is provided for maintaining the emitter element at a thermionic electron emission temperature of the lower work function portions. Free atoms having an ionization potential lower than said given work function are supplied at the surface of the emitter element where positive ions are produced by contact ionization to neutralize the space charge of the thermionically emitted electrons.

.United States Patent In order to prevent minute localized space charge regions at the emitter surface, the ion neutralization must be provided somewhat homogeneously at the emission surface. This factor establishes a preference of small spacing between the area portion of the emitter from which electron emission occurs and the area portion of the emitter at which contact ionization occurs. According to prior art practices, the only solution to the problem has been to provide a single material emitter which will both emit electrons and contact-ionize atoms. But, as previously stated, this has the disadvantage of being a high temperature device. My invention has proved that such prior art practice is not necessary, provided the sizes of the interspersed high and low-work function surface areas are kept small. By so doing the electron emission and ion generation areas are so tightly integrated that space charge effects are not troublesome at the emission surface- Although it is not necessary to limit area size in order to realize successful operation of my invention, the best results are obtained if the maximum dimensions of the individual high and low work function areas are not greater than about one mil.

In the drawing, the single figure is an axial section view of an electron tube incorporating a preferred embodiment of the invention.

An electron tube 10 incorporating the invention comprises a vacuum tight envelope 12 enclosing a cathode 14 and an anode 16in an atmosphere of ionizable gas atoms. The cathode 14 includes a metal housing 20 which contains a heating coil 22 in one end thereof and an electron emitter element 24 in the other end. The cathode housing 20 may, for example, be made of nickel. The heater coil 22 is conventional and adapted to raise the emitter element 24 to thermionic electron emission temperature as hereinafter described. The emitter element 24 comprises a composition of intermixed particles of at least two different materials having substantially different work functions, thus providing, according to the invention, an emitting surface of interspersed high and low work function areas. Hereinafter the two materials will be described as having either a high work function or a low work function. It will be appreciated that the terms low work function and high work function are relative terms and are so used to describe a material simply as having a work function less or greater than that of another material with which it is compared. The ionizable atoms within the envelope 12 must necessarily have an ionization potential lower than the high work function material of the emitter element 24.

According to a preferred embodiment of the emitter element 24, the required high work function and low work function intermixed particles comprise tungsten and barium-barium oxide, respectively. Such an emitting element may be fabricated by providing tungsten and barium carbonate in powder form having particle sizes preferably not appreciably greater than one mil. These powders are thoroughly mixed together, combined with suitable binders, molded, and sintered according to wellknown practices. Suitable activation serves to convert the barium carbonate particles to barium-barium oxide combinations thus providing one of the so-called, low work-function emitter materials known in the art.

Further, in accordance with a preferred embodiment of my invention, the ionizable atoms are provided as cesium vapor. Such may be accomplished by enclosing a cesium compound within the envelope 12, evacuating the envelope, and processing the cesium compound to produce pure cesium. At ordinary room temperatures cesium vaporizes sufficiently to provide the required amount of free ionizable cesium atoms. As an alternative method of supplying adequate ionizable atoms at the 3 emitter surface, a cesium atom gun, such as disclosed in U.S. Patent 2,841,726 issued to R. C. Knechtli, can be used. Since tungsten has a work function of about 4.5 electron volts and cesium has an ionization potential of about 3.89 electron volts, the required relationship of ionization potential to work function according to the invention is satisfied by the described preferred embodiment. It will be appreciated that the selection of tungsten, barium, and cesium constitutes but a preferred em bodiment and that alternative materials are suitable as will be hereinafter described.

In the operation of the tube electrical power is applied to the two leads 26 of the heating coil 22 to raise the cathode 14 only to a temperature at which the barium-barium oxide surface portions of the emitter element 24 will thermionically emit electrons. This may, for example, be about l300 K., a temperature far below that required to obtain any sizable amount of electron emission from the high work function tungsten portions. However, no electron emission is intended to be obtained from the high work function surface portions of the emitter element. Rather, these portions are provided solely for the purpose of producing positive ions by contact ionization of the cesium atoms. In accordance with the phenomenon of contact ionization, the atom to be ionized must contact or closely approach a material which has a higher work function than the ionization po tential of the atom.

The amount of ion neutralization of the electron space charge is a function primarily of the vapor pressure of the cesium atoms and the cathode temperature. In the case of the preferred electron emitter of tungsten-bariumbarium oxide interspersed surface operated in conjunction with free cesium atoms, a pressure of approximately 10- mm. mercury within the envelope 12 results in sufilcient vaporization of the cesium to produce the required ion generation.

The emitter element 24 has been described in accordance with a preferred embodiment as comprising intermixed tungsten and barium-barium oxide particles. However, many other suitable materials may be substituted providing the required relationship between work functions and ionization potentials are complied with. The vaporizable alkali metals including lithium, sodium, potassium, rubidium, and cesium represent suitable ionizable materials to supply the free gas atoms since they have practical ionization potentials. On the other hand, the materials which constitute the high-work function portion of the emitter element must possess a work function greater than the ionization potential of the free atoms and must have a melting point higher than the temperature to which the emitting element must be raised to produce adequate thermionic emission. Metals such as tungsten, molybdenum, iron, nickel, and the like are therefore suitable. Although high in cost, platinum might be used as the high work function material of the emitter element and radium as the source of free, ionizable atoms.

The selection of the material to constitute the low work function portion of the emitter element is not one which enters into the operability of the invention but only into the practicability of it. called, low work function material of the emitter element should be low solely to permit low temperature thermionic electron emission. Accordingly, any of the known low work function materials may be used. The stated preferred material of barium-barium oxide constitutes but one of many known good emitting materials. For example, mixtures of barium, calcium and strontium oxides are excellent choices for low temperature emitters. Thoriated tungsten is also an excellent emitter of relatively low temperature operation requirement. As indicated on page 40 of the textbook Vacuum Tubes, by Spangenberg, McGraw-Hill, 1948, thoriated tungsten cathodes are usually operated at about 1900 C., and have a work function equal to or greater than 2.6 volts, depending upon the surface coverage of thorium on the tungsten base.

Best results have been obtained using the previouslydescribed emitter made by sintering a mass of intermixed tungsten and barium oxide particles. This is believed to be due to the fact that in the sintered mass of intermixed particles a positive distribution of interspersed minute high and low work function areas is provided. It is such a positively provided surface interspersion which contributes to avoiding the hereinbefore described 10- calized minute space-charge regions at the emission surface. Moreover, as stated above the operating tempera ture of the intermixed tungsten and barium oxide particles is l300 K., which is 600 below the usual operating temperature of thoriated tungsten cathodes.

What is claimed is:

1. In an electron tube having an envelope, a low temperature plasma source including an emitter element within said envelope comprising intermixed particles of tungsten and barium oxide, means for maintaining said emitter at a temperature of about 1300 K., and a quantity of free atoms of an alkali metal having an ionization potential lower than the work function of tungsten in the space adjacent to the surface of said emitter element for producing positive ions at said surface by contact ionization of said free atoms by said tungsten particles.

2. The combination recited in claim 1, wherein the maximum dimensions of said particles is not greater than about one mil.

3. The combination recited in claim 1, wherein said alkali metal is cesium.

4. The combination recited in claim 3, wherein the pressure of said cesium is approximately 10" mm. of mercury.

References Cited in the file of this patent UNITED STATES PATENTS 2,492,142 Germeshausen Dec. 27, 1949 2,556,855 Stutsman June 12, 1951 2,604,603 McLinden July 22, 1952 2,700,000 Levi et al. Jan. 18, 1955 2,718,607 Katz Sept. 20, 1955 2,813,220 Coppola Nov. 12, 1957 2,845,567 Geiger July 29, 1958 The work function of the so- 

